Certain petrochemicals are produced commercially by the partial oxidation of an appropriate hydrocarbon in the vapor phase over a suitable catalyst and in the presence of an oxygen-containing gas. For example, cyclic anhydrides are produced commercially by the vapor phase catalytic partial oxidation of aromatic hydrocarbons, such as o-xylene or benzene, or straight-chain hydrocarbons, such as n-butane, or butene, in the presence of an oxygen-containing gas, over a vanadium-containing catalyst. Similarly, nitriles, alkylene oxides, aldehydes and halogenated hydrocarbons are produced by the partial oxidation of appropriate alkanes and alkenes in the presence of selected catalysts. Air is generally used as the oxygen-containing gas, because of its low cost and ready availability. The reaction can be carried out in any suitable reactor, such as a fixed bed, fluidized bed, moving bed, trickle bed or transport bed reactor, and it produces the petrochemical, and generally carbon monoxide (CO), carbon dioxide (CO.sub.2), water, and smaller amounts of other partially oxidized by-products. The reaction equipment train generally consists of a reactor, in which the petrochemical product is produced, a scrubber, in which the petrochemical product is scrubbed from the reactor effluent gases by means of water or other solvent for the petrochemical, and means for further treating the scrubbed effluent gases.
Currently, it is common to practice the above-described process on a single pass basis with the conversion of hydrocarbon to the desired petrochemical product being maximized. This results in a low overall efficiency, since the selectivity to petrochemical product is below the maximum. Consequently, the scrubber effluent gas contains considerable amounts of CO and CO.sub.2, in addition to unreacted hydrocarbon. These products are usually incinerated, so that the only return realized from them is heat value. In many instances the products are flared, so that even the heat value is not realized. In modified processes, a portion of the scrubber effluent gas is recycled, the conversion of the hydrocarbon feedstock is lowered and the selectivity of hydrocarbon conversion to the desired petrochemical product is increased. The remainder of the effluent is purged from the system to prevent the build-up of CO, CO.sub.2 and nitrogen (introduced into the system when air is used as the source of oxygen). These improvements result in a reduced "per pass" conversion, but the overall efficiency of the process is increased.
Federal Republic of Germany (FRG) Patent Application Disclosure 25 44 972 discloses a maleic anhydride manufacturing process in which the reactor feed comprises C.sub.4 hydrocarbons, air, CO and CO.sub.2. In the process of this patent, maleic anhydride is recovered from the reactor effluent gas stream and a portion of the remaining stream is recycled to the reactor. This patent also teaches recovering butane by temperature swing adsorption (TSA) from the non-recycled gas stream and recycling the recovered butane to the reactor. The butane is desorbed from the adsorbent at elevated temperature by fresh air, and the air-butane mixture is recycled to the reactor.
U.S. Pat. No. 4,231,943 discloses the production of maleic anhydride by the reaction of n-butane and air in the presence of a catalyst comprising vanadium and phosphorus oxides. The process of this patent includes the steps of recovering maleic anhydride from the gaseous oxidation reactor effluent, directly recycling a portion of the maleic anhydride-free effluent to the reactor, separating relatively pure n-butane from the remaining gaseous effluent and recycling the relatively pure n-butane to the feed stream.
U.S. Pat. No. 4,987,239 discloses a process for the production of anhydrides by the partial oxidation reaction of a hydrocarbon with an oxygen-containing gas in the presence of a suitable catalyst. In the process of this patent, the gaseous effluent from the maleic anhydride product scrubber is compressed and sent to a selective separator, e.g. a pressure swing adsorption (PSA) unit containing a hydrocarbon-selective adsorbent, wherein a substantial proportion of the unreacted hydrocarbon contained in the effluent is recovered, and the unreacted hydrocarbon and a controlled amount of a gaseous flame suppressor is recycled to the partial oxidation reactor.
The above patents do not discuss or make allowance for moisture contained in the gaseous effluent from the partial oxidation product recovery unit and in purge air. Moisture is produced in the partial oxidation reaction; accordingly, the hot gaseous effluent from the reactor contains moisture. As the effluent gas passes through the product scrubber some moisture may be removed due to cooling of the gas stream, if an aqueous solvent is used. When a nonaqueous solvent is used moisture is generally not permitted to condense. In any event, the gas stream leaving the scrubber still contains moisture, and in fact can be saturated with moisture, even if a nonaqueous scrubbing agent is used. Moisture is more strongly adsorbed than hydrocarbons and carbon oxides by conventional adsorbents; accordingly, unless the moisture is removed from the gas stream entering the adsorption units, it will be preferentially adsorbed onto the adsorbent, thereby significantly reducing the capacity of the adsorbent for hydrocarbon adsorption.
The problem of moisture is further aggravated when ambient air is used as a purge gas for regeneration of the beds of adsorbent. Ambient air contains moisture; thus, moisture will replace the hydrocarbon being desorbed from the adsorption beds during the purge step when the beds are purged with the air. This will further reduce the capacity of the adsorbent during the adsorption step of the following cycle. Unless an additional driving force, such as reduced pressures and/or elevated temperatures are employed in the bed regeneration procedure, water will accumulate in the beds and may render the process inoperable.
Because of their industrial importance, recycle partial oxidation processes in which problems such as those noted above are eliminated or minimized are constantly sought. The present invention provides a recycle partial oxidation process which prevents moisture from accumulating in the hydrocarbon-selective adsorbents used in hydrocarbon adsorption-based recycle partial oxidation processes.